Final cost is ~$1,127. For improved performance you can chuck in another 6870 down the line. Install Windows and ARMA II on the SSD. This should provide great performance in ARMA II running at 1440x900 as well as your other games. If you want to reduce costs you could look into a cheaper PSU as the one I listed has a bit of a premium on it since it is modular (you can remove the cables you don't need), you could also remove the SSD however personally I'd keep it. The CPU is fully unlocked and ready to overclock, however I haven't added an aftermarket cooler to the build; I personally recommend the Corsair H70. If you do not ever want to overclock you can get the cheaper non 'K' version of the processor and save a little money.
The motherboard I have listed is a little more expensive than I normally suggest. To be honest, I didn't put too much effort in as when ever I list one someone claims something bad about it and never bothers to post a source. It supports dual graphic cards, if that doesn't interest you, you can save a couple of dollars and get a cheaper motherboard without it.
Either way, this is a starting point. Let's see where this goes.

Yes, you could cheap out on some components and still have pretty much the same in-game performance, however, the trade-off would be future upgradability; a performance hit to multitasking, desktop experience and loading times; and/or case airflow, aesthetics and cable management.
Three areas where we could cut down in costs is:

PSU

As I said above, I've selected a modular PSU. This is considered a premium part and carries a higher price over non-modular designs. You could save some money by switching to another model such as corsair's tx650. Corsair's products also tend to be a little more expensive so you may consider switching to another brand. The current model is a 650W PSU; This is perfect for dual 6870s. If you do not plan to ever upgrade to a dual card setup you can choose a cheaper 500W model instead.

Keep in mind, try not to be too cheap when choosing a PSU. While it does not directly affect performance of games, it is what is powering those components that are; if you've selected a PSU that is of inferior quality, you may end up with decreased performance, extra heat and potentially damaging or destroying your other components. The quality of your PSU also has a big impact on overclocking as factors such as good voltage regulation (not much voltage variance at different load levels) will allow you to set a lower voltage to achieve the same overclock and thus less heat.

Motherboard

Again, you may be able to find cheaper models. From memory this board has USB 3.0 (internal & external), Sata 3 and two PCI-e 3.0 lanes. Cheaper models usually means less features. If you do not plan to have a second graphics card you will not require the second PCI-e lane. Some boards offer better overclocking and more overclocking features than others, however you do not need anything too fancy for this. Most people will only need the very basic options.
Look out for reviews for any particular boards. You do not need to read the whole thing, simply reading the conclusion will generally give you a good enough idea if there's any issues with the board.

SSD

Again a premium product, and the easiest place to save some money from. It has benefit in mostly multitasking, desktop experience and loading times. If you don't maintain your PC very well you'll probably see massive benefit to this as well, since you'll probably have several different background programs and services running which will be accessing your hard drive/s from time to time; This would be a nightmare on a single mechanical hard drive, however the addition of a SSD will amend this depending on it's implementation.
If you install ARMA II on the SSD it will also have gains. While it doesn't exactly have a constant fps improvement it alleviates certain situations of stuttering and lod thrashing.

While personally I wouldn't go without one myself nowadays (as it greatly improves my desktop experience which is where I spend the majority of my time), you can save a good fair deal of money if you drop it from the build.

Two other areas that you could save money would be RAM & GPU. While I'm not suggesting getting a weaker graphics card, you might be able to find better deals; different models or even the exact model I've listed depending on which company the board has been branded under.

Personally I wouldn't recommend it, but you could drop down to 4GiB of ram. This shouldn't make much difference in the short term and it's easily enough expanded upon in the future. Though you'd have to ask is saving ~$21 that important.

Well it's simple enough. Search for something along the lines of "1155 Motherboard" in newegg. 1155 is the number of pins in the H2 socket which sandy bridge and Ivy Bridge use.

A close-up picture of an LGA 1155 (H2) socket on a motherboard.

From that list find motherboards that contain "p67" or "z68" in their names; these are known as chipsets. Chipsets basically tell you several features and functions that the board will have. Both of the above chipsets allow overclocking. The z68 has all the functions of the p67 and a couple of extra; you probably won't use these anyway. z68 boards will be slightly more expensive than their p67 counterparts.
Put all the z68 and p67 boards that are within your budget in a separate browser window. Once you have enough, look through their general features. Eliminate whichever do not have the features you are looking for such as multiple PCI-e slots if you plan to have multiple graphics cards, SATA 3 (6Gb/s not 3Gb/s) and/or USB 3.0 etc.
When you only have motherboards that offer all the features you want, take a look at the newegg review section and proper full blown reviews on review sites (you only need to read the conclusion if you want to), do this starting at the least expensive board; this step is quality control, you are looking for issues.
Remember, people are much more likely to post a review on newegg if they are unsatisfied; if a product has a high volume out and yet only a few reviews, if the majority of them are negative, this doesn't mean you are likely to encounter this issue (further research this issue).

Take the cheapest board that has all the features that you want and passes your quality control check.

Final cost is ~$1,127. For improved performance you can chuck in another 6870 down the line. Install Windows and ARMA II on the SSD. This should provide great performance in ARMA II running at 1440x900 as well as your other games. If you want to reduce costs you could look into a cheaper PSU as the one I listed has a bit of a premium on it since it is modular (you can remove the cables you don't need), you could also remove the SSD however personally I'd keep it. The CPU is fully unlocked and ready to overclock, however I haven't added an aftermarket cooler to the build; I personally recommend the Corsair H70. If you do not ever want to overclock you can get the cheaper non 'K' version of the processor and save a little money.
The motherboard I have listed is a little more expensive than I normally suggest. To be honest, I didn't put too much effort in as when ever I list one someone claims something bad about it and never bothers to post a source. It supports dual graphic cards, if that doesn't interest you, you can save a couple of dollars and get a cheaper motherboard without it.
Either way, this is a starting point. Let's see where this goes.

Exiled.

:
This is a pretty good setup, but a few things don't follow good PC building guidelines (save money/future proof).